1. Field of the Invention
This invention relates to a high consistency heat-curable silicone rubber composition and a silicone rubber obtained by curing said composition. The heat-curable silicone rubber composition is cured into a cured product having a high strength and can be used for many diverse applications, for example, tubes, gaskets, diaphragms and the like.
2. Description of the Prior Art
The prior art teaches many ideas that relate to silicone rubber compositions including those having optical clarity. These silicone rubber compositions can be used for many diverse applications requiring optical clarity, for example, tubes, hoses, diaphragms, packings, contact lenses, etc.
It is necessary to blend an extrusion compatible crosslinker into silicon rubber compositions prior to the extrusion process and the subsequent curing process. Dichlorobenzoyl peroxide is a well-known extrusion compatible crosslinker which is widely used in such silicone rubber compositions. It is an unstable peroxide in compositions and initiates a free radical reaction that forms crosslinkages between adjacent polymer chains. However, dichlorobenzoyl peroxide has a disadvantage of needing to go through a lengthly post-cure process to remove harmful decompositional by-products.
As compared with the above peroxide curing compositions, addition catalyzed compositions do not require the time consuming post-cure process, but typically do, to obtain optimum physical characteristics. The prior art teaches many ideas that relate specifically to high consistency silicone rubber compositions which are catalyzed by hydrosilation reactions and cured into silicone rubbers with optical clarity.
Addition reaction occurs between hydrosilyl groups and vinyl groups, usually catalyzed by a platinum compound such as chloroplatinic acid, forming Si--C bonds as described in the following formula. ##STR1## This type of crosslinking system has some advantages over other kinds of vulcanizing agents. Once the proper components are allowed to react together, the composition can be vulcanized rapidly at elevated temperatures or much more slowly at room temperature.
Among silicone rubber compositions, fluorosilicone rubber compositions such as those containing trifluoropropyl groups can be cured in the presence of a hydrogen atom-containing crosslinker as the other silicone rubber compositions can. The fluorosilicone rubber compositions have less heat and acid resistance, but have improved physical properties as compared with dimethylsilicone rubber compositions.
U.S. Pat. No. 3,671,480, issued to Wada, discloses addition crosslinked polyorganosiloxane compositions which contain two different vinyl-containing polydiorganosiloxanes to achieve notable improvements in the physical properties, especially tear strength. One of the two polymers is a gum having a polymerization degree greater than 3000 and containing from 0.02 to 0.2 mole percent of vinyl radicals. The second polymer utilized contains at least 2 mole percent of vinyl radicals with a polymerization degree of less than 100, which encompasses only liquid polydiorganosiloxanes.
U.S. Pat. No. 4,857,564, issued to Maxson, also teaches high consistency organosiloxane compositions which contain two different polydiorganosiloxanes. Maxson teaches improved tear strength and elongation are achieved by including in the composition a vinyl-containing liquid polydiorganosiloxane that is immiscible with the gum type polydiorganosiloxane composed of the major organosiloxane ingredient of the composition.
The prior art teach different ways with which one can improve the physical properties of addition catalyzed polydiorganosiloxane vulcanizates. The patents issued to Wada and Maxson, for example, utilize a blend of two different polydiorganosiloxanes to achieve desired results. The compositions catalyzed by hydrosilation reactions can display outstanding tear strengths when two different vinyl-containing polydiorganosiloxanes are blended together. A majority of the composition contains a lower vinyl-containing organosiloxane, and a higher vinyl-containing organosiloxane is added thereto in a smaller quantity to create an uneven vinyl distribution. This facilitates localized crosslinking networks which help to improve tear strengths of the material.